Strydom W J, Korf G J
Medical University of Southern Africa, Department Medical Physics.
Med Dosim. 1995 Summer;20(2):135-8. doi: 10.1016/0958-3947(95)00012-l.
To convert an ionization measurement to absorbed dose, the TG-21, the IAEA, and the TG-25 protocols for the calibration of high energy electron therapy machines demand an accurate knowledge of the mean restricted collision mass stopping power ratio, (L/rho)medair, and the electron fluence correction factor, Prepl. This paper presents a semi-analytical expression to calculate (L/rho)medair at depths in the range of 0 to 30 g/cm2 and for any energy in the range of 4-60 MeV, for water, polystyrene, and acrylic relative to air, and another expression to calculate Prepl for any cylindrical ionization chamber with an inner diameter in the range of 3 to 7 mm. The published values for (L/rho)medair were fitted to a single analytical expression with 10 coefficients using a nonlinear least square routine. Another expression with 4 coefficients was used to fit the electron fluence correction factor, Prepl, for cylindrical ionization chambers, as a function of chamber diameter and the mean electron energy at depth of measurement. In the electron energy range of 4-60 MeV, and for all published depths, the calculated (L/rho)medair values agree with the published values within +/- 1% in 99% of the cases. The calculated Prepl values agree with the tabulated values within +/- 1% in all of the cases. The proposed equations now allow one to use a computer to calculate the values of (L/rho)medair and Prepl accurately, doing away with the time-consuming procedure of interpolating between values from tables during the routine calibration of high energy electron therapy machines.
为了将电离测量值转换为吸收剂量，针对高能电子治疗机的校准，TG - 21、国际原子能机构（IAEA）以及TG - 25协议要求精确知晓平均受限碰撞质量阻止本领比（L/rho）medair和电子注量修正因子Prepl。本文给出了一个半解析表达式，用于计算水、聚苯乙烯和丙烯酸相对于空气在深度范围为0至30 g/cm²以及能量范围为4 - 60 MeV内任意能量下的（L/rho）medair，还给出了另一个表达式，用于计算内径范围为3至7 mm的任意圆柱形电离室的Prepl。已发表的（L/rho）medair值使用非线性最小二乘法例程拟合为具有10个系数的单个解析表达式。使用具有4个系数的另一个表达式将圆柱形电离室的电子注量修正因子Prepl拟合为室直径和测量深度处平均电子能量的函数。在4 - 60 MeV的电子能量范围内以及所有已发表的深度下，在99%的情况下，计算得到的（L/rho）medair值与已发表的值在±1%范围内一致。在所有情况下，计算得到的Prepl值与表格值在±1%范围内一致。所提出的方程现在允许人们使用计算机准确计算（L/rho）medair和Prepl的值，从而在高能电子治疗机的常规校准过程中省去在表格值之间进行插值这一耗时的程序。
